Previous argument inserted at the wrong point when writing a subshell

Normal operation:

```
$ echo one
$ echo [alt-up]
# command line is now "echo one"
```

But if you're writing a subshell, the argument gets put in the wrong place:

```
$ echo one
$ echo (echo [alt-up]
# command line is now "echo one(echo", and the cursor is over the "("
```
